The present invention relates to a method for generating a head switching signal in a video cassette recorder and an apparatus therefor, and more particularly, to a method for generating a head switching signal in a video cassette recorder and an apparatus therefor, in which the head switching signal is accurately generated even though a vertical sync signal is not detected from a reproduction signal from a video tape.
Recently, video cassette recorders (VCRs) have become compact and light and at the same time a head drum is being rapidly miniaturized. As a result, it has been difficult to accurately control the position of a video head with respect to a pulse generator (PG) signal on a head drum. Thus, it is required that a phase be precisely controlled for each deck. To automatically do so, U.S. Pat. No. 5,357,382 issued on Oct. 18, 1994 proposed an apparatus and method for automatically controlling a head switching position by using a vertical sync signal in a VCR. Referring to FIGS. 1 and 2A through 2D, the technology of the above U.S. patent will be briefly described.
An amplifier 1 of FIG. 1 receives a drum pulse signal PG of FIG. 2A from a head drum (not shown), amplifies the received drum pulse signal PG by a predetermined amount of amplification, and applies the amplified pulse signal to a mono-stable multivibrator 2 as a trigger signal of FIG. 2B. The mono-stable multivibrator 2 is triggered at an edge of the amplified pulse signal output from the amplifier 1 to thereby output a pulse signal delayed by a time constant determined by a resistor R11 and a capacitor C11. A frequency generator 3 generates a frequency signal of 30 Hz which is output as a head switching signal HSw of FIG. 2C. A microcomputer 6 includes an input end INT1 for receiving a vertical sync signal Vsync of FIG. 2D and an input end INT2 for a head switching signal HSw, and judges whether the vertical sync signal Vsync is received at the point of time lagging 6.5H, that is, 412 .mu.sec from an edge of the head switching signal HSw. Here, H represents a horizontal sync period. In other words, the microcomputer 6 judges whether or not an interval "A" between an edge position of the head switching signal HSw and a falling edge of the vertical sync signal Vsync corresponds to 6.5H. The digital delay data obtained by such a judgement of the microcomputer 6 is converted into analog data in a digital-to-analog converter 4. The obtained voltage Vin is altered into a voltage V.sub.A by the resistor R11 and the capacitor C11. The voltage V.sub.A alters the time constant of the mono-stable multivibrator 2. Accordingly, the mono-stable multivibrator 2 delays the output signal of the amplifier 1 by the amount corresponding to the altered time constant, and outputs the delayed result. The delayed signal is output as a head switching signal HSw of 30 Hz by the frequency generator 3. The microcomputer 6 stores the delayed data in an EEPROM 5 if the interval "A" between the vertical sync signal and the head switching signal of 30 Hz is 6.5H, and reads out the stored data as necessary to output the delayed data. Accordingly, an accurate head switching signal HSw can be output.
However, since a digital recording-type VCR such as a standard definition digital VCR does not record a vertical sync signal on a video tape but uses a predetermined track pattern, an accurate head switching signal cannot be obtained using the technology disclosed in the above-described patent. Even though a vertical sync signal is recorded on a video tape, if an interval between a head switching signal and the vertical sync signal is beyond a predetermined value, for example, 6.5H, an accurate head switching signal cannot be output.